High Tc superconducting asymmetric gradiometer for biomagnetic applications

Abstract

We describe a high transition temperature superconducting, first-order gradiometer intended for biomagnetic measurements in an unshielded environment. The gradiometer involves a single-layer, planar flux transformer with two loops of unequal size, the smaller of which is inductively coupled to the pickup loop of a directly coupled magnetometer. In this configuration, the presence of the flux transformer reduces the sensitivity of the magnetometer by only about 5%. The flux transformer is patterned in a thin film of YBa2Cu3O7-δ deposited on a 100 mm diam wafer, and has a baseline of 48 mm. The flux transformer and magnetometer substrates are permanently bonded together in a flipchip arrangement. The common mode rejection of uniform magnetic field fluctuations in any direction is better than 1 part per 100. The outputs of two such gradiometers are subtracted to form a second-order gradiometer, which rejects first-order gradient fluctuations to about 1 part in 100. With the aid of three orthogonally mounted magnetometers, one can reduce the response of the gradiometers to uniform field fluctuations to below 100 ppm. This system is used to detect magnetic signals from the human heart in an unshielded environment. © 2000 American Institute of Physics.